Because it’s incredibly difficult a priori to restrict only research that could lead to unethical eugenics without restricting that which could lead to ethically sound, widely beneficial eugenics or even unrelated advances.
You can (for example) ban research on things might lead to the development of novel poisons. But that ban is almost sure to prevent the development of some new cancer treatments.
Anything that increases the health outcomes of humans in the long-run would be broadly beneficial and could stem from research that's initially targeted at understanding the role genetics plays in intelligence. That could be breakthroughs in early childhood (or lifelong) nutritional support, eradication of diseases, creating lower susceptibility to some forms of cancer or heart disease, improvements in pedagogy, improvements in general intelligence, lower susceptibility to cognitive decline.
Research is not laboring to progressively color in a pre-defined picture, where you can just tell research scientists "don't color in the naughty bits".
Do you have a list of all the genes and all the things that they impact, even slightly, alone and in combinations? That list in itself would seem to be extraordinarily valuable, perhaps even Nobel Prize winning.
We don't know what unrelated items or generally applicable patterns we might find when we go looking into basic research.
You can (for example) ban research on things might lead to the development of novel poisons. But that ban is almost sure to prevent the development of some new cancer treatments.